Here’s a simple thing that I ran across. I have a vendor that recommended that I set the Maximum memory in my LPARs to the system maximum. That way you never have to reboot to increase the maximum memory in that LPAR. I found out later that setting your LPARs memory to the system maximum makes the hypervisor allocate more memory for overhead.
This is a very old configuration issue, but I just ran across the actual numbers. When the LPAR is activated, the hypervisor allocates 1/64th the LPAR maximum for page frame tables. This is a memory structure that the hypervisor uses to track the memory pages used by the LPAR. So, lets say you have a 128GB managed system with LPARs that only really need 16GB of RAM, but the LPAR’s maximum memory is set to 128GB. By the time you’ve activated your 7th LPAR your using 2GB per LPAR, or 14GB of RAM, just for the hypervisor memory page frame tables.